The applicants have proposed an X-ray CT method and apparatus in which conical X-ray beams with remarkably small sectional radiation area are locally radiated only on a local region, a part of an object, to obtain clear three dimensional X-ray absorption coefficient data on the region. For example, JP-A-2000-139902 discloses its CT method and apparatus.
In this laid open publication, the applicants have proposed a basic X-ray CT method in which a rotation center of a rotary arm with an X-ray generator and an X-ray detector faced each other was set on a center of a local region of an object, conical X-ray beams are locally radiated all around or half around a circumference of the object to obtain transmitted data, and the obtained transmitted images are backprojected to obtain three dimensional X-ray absorption coefficient data of the local region. Also they have proposed a method to obtain an X-ray panoramic image of a dental arch in the dental field using the above-mentioned CT method in which a clear X-ray panoramic image could be obtained with a small amount of X-ray exposure and a short radiation time.
However, such obtained X-ray panoramic image included obstacle shades such as a neck bone which existed on the radiation orbit of a conical X-ray beam other than the dental arch like a prior film-type X-ray panoramic images. Therefore, improvement of clearness of images has been desired. Further, the panoramic images were generally used images for diagnosis in the dental field, however they weren't easily understood by intuition as a dental arch was actually seen from one direction. Although the condition of a dental root could be understood by the obtained X-ray panoramic images, it was difficult to distinguish where the root exists from a cheek to a tongue.